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Environmental Processes
Electron-donating Phenolic and Electron-accepting Quinone Moieties in Peat Dissolved Organic Matter: Quantities and Redox Transformations in the Context of Peat Biogeochemistry Nicolas Walpen, Gordon James Getzinger, Martin H. Schroth, and Michael Sander Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00594 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018
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Environmental Science & Technology
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Manuscript
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Electron-donating phenolic and electron-accepting quinone moieties in peat
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dissolved organic matter: quantities and redox transformations in the context of
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peat biogeochemistry
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NICOLAS WALPEN, GORDON J. GETZINGER, MARTIN H. SCHROTH, AND MICHAEL SANDER*
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Institute of Biogeochemistry and Pollutant Dynamics,
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Department of Environmental Systems Science,
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Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
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Submitted as manuscript to Environmental Science & Technology
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*Corresponding author: Michael Sander Email:
[email protected] Phone: +41 44 632 83 14
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Number of pages: 29 Number of figures: 4 (total of 1800 word equivalents) Number of tables: 0 Number of words: 5789 Total Number of word equivalents: 7589
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ABSTRACT
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Electron-donating phenolic and electron-accepting quinone moieties in peat
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dissolved organic matter (DOM) are considered to play key roles in processes
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defining carbon cycling in northern peatlands. This work advances a flow-injection
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analysis system coupled to chronoamperometric detection to allow for the
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simultaneous and highly-sensitive determination of these moieties in dilute DOM
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samples. Analysis of anoxic pore water and oxic pool water samples collected across
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an ombrotrophic bog in Sweden demonstrated the presence of both phenolic and
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quinone moieties in peat DOM. The pore water DOM was enriched in phenolic but
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not quinone moieties compared with commonly used model aquatic and terrestrial
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DOM isolates. Significantly lower phenol content in DOM from oxic pools than
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DOM from anoxic pore waters indicated oxidative DOM processing in the pools.
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Consistently, treatment of peat DOM with laccase, a phenol-oxidase, under oxic
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conditions resulted in irreversibly removal of phenols and reversible oxidation of
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hydroquinones to quinones. Electron transfer to peat DOM was fully reversible over
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an electrochemical reduction and subsequent O2-reoxidation cycle, supporting that
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quinones in peat DOM serve as regenerable microbial electron acceptors in peatlands.
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The results advance our understanding of redox processes involving phenolic and
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quinone DOM moieties and their roles in northern peatland carbon cycling.
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TOC ART
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INTRODUCTION
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Northern peatlands store an estimated 500 ± 100 Pg of carbon1 in the form of
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peat organic matter (OM) and therefore represent a significant global carbon pool.
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OM has accumulated in these systems since the end of the last glaciation due to slow
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plant material decomposition1 resulting from water-logged, anoxic conditions below
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the peat surface and low average annual temperatures.2-5 Northern peatlands include
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ombrotrophic bogs characterized by oligotrophy and low pH, two factors that further
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contribute to slow OM decomposition. Because of the large amount of carbon stored
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in northern peatlands, it is critical to understand processes that perturb carbon
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balances in these systems. This is particularly true for processes that lead to the
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release of stored carbon as carbon dioxide and methane, with potentially severe
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implications for global warming,6-8 and as dissolved organic matter (DOM) into
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receiving watersheds.9,10
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Two processes that affect carbon cycling in northern peatlands center around
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electron transfer reactions from electron-donating phenolic and to electron-accepting
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quinone moieties in peat DOM, respectively. The first process is the oxidation of
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phenolic peat DOM (hereafter referred to as ‘phenols’ which include mono-
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hydroxylated aromatics as well as hydroquinones). These phenols are produced by
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Sphagnum mosses11,12 —the dominant peat-forming plant genus in northern
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peatlands2,4— and are released, either actively or during moss decomposition, to peat
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pore waters where they purportedly inhibit extracellular hydrolases and thereby slow
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OM decomposition.13-16 In anoxic pore waters, low activities of phenol-oxidizing
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(per)oxidases, which require oxygen or peroxide as co-substrates, render phenols
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stable.17 The so-called ‘enzymic latch’ hypothesis posits that increased (per)oxidase
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activity caused by more frequent peat oxygenation events through water table draw-
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down in a warming climate14 could remove phenols and thus their inhibitory pressure
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on hydrolase activity, thereby triggering OM decomposition and thus significant
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releases of carbon from northern peatlands.14,18-20
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The second process is the reduction of quinone to hydroquinone moieties
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(hereafter referred to as ‘quinones’) in peat DOM: quinones may be used as electron
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acceptors for anaerobic microbial respiration,21-25 including anaerobic methane
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oxidation.26-29 As a result, quinone reduction is considered to significantly suppress
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methane emissions from northern peatlands.5,30-34 This suppression may be
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sustainable over longer time periods if the formed hydroquinones are oxidized back to
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the electron-accepting quinones through electron transfer to oxygen at anoxic-oxic
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interfaces in the peat35 or to peat particulate OM, akin to DOM electron shuttling in
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dissimilatory iron oxide reduction.22,36,37 While reversible electron transfer to peat
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DOM would thus have large implications for methane emissions from northern
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peatlands, this process has not yet been experimentally demonstrated.
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Despite the key role of phenols and quinones in the above processes, little
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information was heretofore available on the quantities and redox transformations of
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these moieties in peat DOM. Obtaining this information has, to a large part, been
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impaired by the lack of a sufficiently sensitive analytical technique to quantify these
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moieties
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chronoamperometric analyses in electrochemical cells, the heretofore most sensitive
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approach to quantify phenols24,38 and quinones23,24, require samples with DOM
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concentrations above those commonly found in natural systems. To supersede these
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analytical challenges, we recently introduced a flow-injection analysis (FIA) system
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to quantify phenols in DOM samples containing only a few milligrams carbon per
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liter.39 We used the FIA system to quantify phenols in peat DOM in a small set of
in
samples
with
low
DOM
concentrations.
Even
mediated
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water samples collected in three Swedish ombrotrophic bogs and to monitor peat
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DOM enzymatic oxidation.39 However, the inability of the original FIA system to
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quantify the number of electrons transferrable to DOM precluded a comprehensive
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assessment of peat DOM redox properties and transformations, including the
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quantification of electron accepting quinones and the analysis of electron transfer
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reversibility during peat DOM oxidation and reduction.
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The objectives of this work were (i) to extend the previously-introduced FIA
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system for additionally quantifying the electron-accepting moieties in dilute DOM
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samples and to validate and assess the performance of the extended system, (ii) to
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systematically characterize variations in the geochemical and redox properties of peat
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DOM samples collected across a peatland and to compare these redox properties with
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those of well-characterized model DOM isolates, and (iii) to assess the reversibility of
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electron transfer from and to phenols and quinones in peat DOM during enzymatic
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oxidation and over an electrochemical reduction and O2-reoxidation cycle,
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respectively. For objectives (ii) and (iii), we collected and analyzed 29 pore and 30
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pool water DOM samples from an ombrotrophic bog in central Sweden. This work
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provides heretofore missing information on the quantities and the transformations of
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phenols and quinones in peat DOM and thereby advances a more holistic
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understanding of redox processes involving peat DOM that impact carbon cycling in
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northern peatlands.
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MATERIALS AND METHODS
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Chemicals and enzymes. Potassium chloride, potassium phosphate dibasic
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trihydrate, potassium phosphate monobasic, sodium acetate trihydrate, acetic acid,
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(+)-sodium
L-ascorbate,
disodium
9,10-anthraquinone-2,6-disulfonate,
and
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diammonium
2,2'-azino-bis(3-ethyl-6-benzothiazolinesulfonate)
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obtained
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(Zwitterionic viologen, ZiV) was synthesized and purified as described elsewhere.40
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Laccase from Trametes versicolor was obtained from Fluka (activity: 22.4 U·mg-1;
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product number 53793) and used as received. Aqueous laccase stock solutions (5
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U·mL-1) were freshly prepared for each experiment.
from
Sigma-Aldrich.
(ABTS)
were
N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium
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Solutions. All solutions were prepared in ultrapure water (resistivity >18.2
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MΩ·cm) obtained from a Barnstead Nanopure Diamond system. FIA system carrier
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solutions were buffered with 0.1 M phosphate or 0.1 M acetate for analyses at pH 7
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and 5, respectively. The FIA reagent solutions contained the chemical reductant ZiV•-
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(i.e., reduced ZiV in 1 mM phosphate, pH 7) and the oxidant ABTS•+ (i.e., oxidized
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ABTS in 1mM acetate, pH 3.9), respectively, and 0.1 M KCl as electrolyte. All
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solutions were N2-sparged for two hours before transferring into the anoxic glove box
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(N2 atmosphere,
